Homo-Cope-Type Five-Carbon Ring-Expansion Reaction via Silyl Enol Ether

Abstract

A five-carbon ring-expansion reaction from a cyclohexane derivative fitted with a 3-hydroxy-2-silyloxyprop-1-en-1-yl unit in lieu of one of the C=C double bonds in the Cope rearrangement was studied to synthesize cycloundecanone derivatives. It was found that (E)-triethylsilyl enol ether is necessary for the reaction. The trans-isomer with respect to the cyclohexane ring afforded (2E,8E)-cycloundeca-2,8-dien-1-one, while its cis-isomer afforded (2E,8E)- and (2E,8Z)-isomers in a 1:3 ratio.

Key words

ring expansion - carbocycles - silicon - enol ethers - Cope rearrangement

References and Notes

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• 16 Compound 15: Oil. IR (neat): 1658 cm^-1. ¹H NMR (CDCl₃): δ = 0.87-0.96 (1 H, m), 0.89 (9 H, s, t-Bu), 1.15 (1 H, dddd, J = 2.3, 3.5, 12.5, 14.6 Hz), 1.54 (1 H, dt, J = 12.5, 10.0 Hz), 1.93 (1 H, ddt, J = 1.4, 9.5, 12.5 Hz), 2.07 (1 H, ddt, J = 5.6, 14.6, 2.5 Hz), 2.17-2.37 (3 H, m), 2.44-2.54 (2 H, m), 2.81 (1 H, dt, J = 2.5, 11.3 Hz), 5.08-5.20 (1 H, m, CH=CH), 5.30 (1 H, br dt, J = 15.7, 7.5 Hz, CH=CH), 5.90 (1 H, br d, J = 16.1 Hz, CH=CHCO), 6.45 (1 H, ddd, J = 6.0, 9.4, 16.1 Hz, CH=CHCO). ¹³C NMR (CDCl₃): δ = 27.14 (3 C), 30.07, 31.07, 34.20, 34.42, 35.03 (br), 38.86, 49.56, 129.12 (br), 132.31, 136.33, 156.27, 202.97. HRMS: m/z calcd for C₁₅H₂₄O (M⁺): 220.1827; found: 220.1812

14

Five-Carbon Ring Expansion: In a 100-mL round-bottomed flask, a solution of 3c (114.4 mg, 0.386 mmol) in anhyd CH₂Cl₂ (40 mL) was prepared at -60 °C under Ar. To this was added 2,6-lutidine (0.13 mL) and Tf₂O (0.1 mL) successively with stirring; stirring was continued for 1 h at the same temperature. A sat. aq solution of NH₄Cl was added and the mixture was extracted with CH₂Cl₂. The organic layer was dried and the solvent was evaporated to afford an oily residue, which was chromatographed on silica gel (3 g; hexane-EtOAc, 95:5) to give 11 (43.7 mg, 69%).

15

Compound 11: Oil. IR (neat): 1657 cm^-1. ¹H NMR (CDCl₃): δ = 1.47-1.80 (4 H, m), 2.01-2.10 (2 H, m), 2.21-2.30 (4 H, m), 2.61-2.67 (2 H, m), 5.03 (1 H, dt, J = 15.5, 7.6 Hz, CH=CH), 5.31 (1 H, dt, J = 15.5, 7.7 Hz, CH=CH), 5.90 (1 H, d, J = 16.2 Hz, CH=CHCO), 6.31 (1 H, dt, J = 16.2, 7.5 Hz, CH=CHCO). ¹³C NMR (CDCl₃): δ = 27.28, 28.53, 30.07, 34.21, 34.45, 38.59, 128.81, 132.01, 137.38, 156.96, 202.88. HRMS: m/z calcd for C₁₁H₁₆O (M⁺): 164.1201; found: 164.1235.

17

Compound 19: Oil. IR (neat): 1672 cm^-1. ¹H NMR (CDCl₃): δ =1.64-1.74 (4 H, m), 2.17-2.31 (6 H, m), 2.53-2.57 (2 H, m), 5.36-5.43 (1 H, m, CH=CH), 5.52 (1 H, ddt, J = 10.5, 8.2, 1.7 Hz, CH=CH), 6.19 (1 H, dt, J = 16.2, 1.5 Hz, CH=CHCO), 6.80 (1 H, dt, J = 16.2, 6.7 Hz, CH=CHCO). ¹³C NMR (CDCl₃): δ = 22.92, 25.10, 25.15, 28.50, 32.10, 39.81, 128.81, 129.73, 131.57, 151.11, 203.56. HRMS: m/z calcd for C₁₁H₁₆O (M⁺): 164.1201; found: 164.1221.